The present invention relates to a semiconductor memory including a so-called “differential-type cell” with two bit cells for storing opposite logic states to each other, respectively, for amplifying a difference between data stored in one of the bit cells and data stored in the other and outputting the amplified difference as readout data.
Recently, with reduction in the size of fabrication processes, the thickness of transistor oxide films has been reduced more and more. Because of this, in a known memory cell, a leakage voltage and the like are generated in a gate oxide film of an MOS transistor and, due to the leakage voltage and the like, data holding properties are deteriorated. In a data determination method in which data determination is performed by comparing a voltage stored in a memory cell to a threshold voltage, it is difficult to suppress reduction in reliability resulting from the reduction in the size of fabrication processes.
To cope with this problem, a so-called “differential-type cell” including two bit cells and a differential amplifier has been already devised (see Japanese Laid-Open Publication No. 3-120759). In a data write operation, different data indicating opposite logic states to each other are stored in the two bit cells, respectively, for example, according to the levels of respective threshold voltages of the bit cells. On the other hand, in a readout operation, the differential amplifier reads respective potentials of the two bit cells and a difference between the potentials is amplified and then output as readout data.
The differential-type cell is less influenced by a leakage of electric charges, compared to the data determination method in which data determination is performed by comparing a stored voltage in a memory cell to a threshold, so that a large noise margin can be provided. Therefore, a semiconductor memory with excellent data holding properties can be achieved.